In olefin polymerization, especially gas phase polymerization, the rate of heat generation is directly related to the production rate of polymer. The production rate is typically controlled by adjusting the rate of catalyst injection and the concentration of monomer(s) in the reaction zone. The rate of catalyst injection changes the reaction rate and thus the rate at which heat is generated in the bed.
However, the catalyst should be dispersed uniformly throughout the reaction materials to promote uniform polymerization. An effective dispersal of catalyst within the reactor avoids fouling and promotes uniform, consistent production of polymer product. Therefore, the catalyst cannot simply be dumped into the reactor at a faster rate without any consideration for dispersion within the reaction materials. In other words, increased catalyst addition does not increase production rate if not properly dispersed.
Liquid catalysts are commonly used in gas phase polymerization. Liquid catalysts require less equipment and raw materials to make. Liquid catalysts also impart fewer impurities to the final polymer product. Further, the activity of liquid catalysts is not adversely influenced by the surface area of a support material. However, the proper dispersal of liquid catalysts into a reactor is difficult.
An effective dispersal of liquid catalyst within the reactor presents several challenges. For example, the liquid catalyst is typically soluble in the reaction medium and can deposit on the resin or polymer forming in the reactor, accelerating polymerization on the surface of the particles of the bed. As the coated resin particles increase in size, the particles are exposed to a higher fraction of catalyst solution or spray because of the increased cross-sectional dimensions. If too much catalyst is deposited on the polymer particles, the polymer particles can grow so large that the particles cannot be fluidized, causing the reactor to be shut down.
Further, the initial polymerization rate upon liquid catalyst injection to the reactor can be so high that the newly formed polymer or resin particles can soften or melt. Such softened or melted polymer can adhere to one another and form larger particles in the fluidized bed. Such large particles are difficult to fluidize and consequently, plug the reactor, requiring the reactor to be shut down.
Conversely, poor dispersion of the catalyst can cause entrainment. Entrainment can occur if the polymer particle size is too small. Entrained particles can foul recycle lines, compressors, and coolers. Entrained particles can also increase static electricity which can cause sheeting in the reactor.
Moreover, newly developed catalysts with high catalytic activity present many new challenges. Such new catalysts typically have high initial activity and polymerize before being dispersed in the reactor bed. As such, these highly active catalysts are even more prone to unwanted agglomerate formation and fouling.
Various nozzles have been proposed to inject and better disperse liquid catalyst into reactor systems. For example, U.S. Pat. No. 4,163,040 discloses a catalyst spray nozzle that utilizes a biased valve member to regulate catalyst flow. U.S. Pat. No. 5,693,727 discloses a catalyst spray nozzle that utilizes a shroud about a central injection tube. Each of those nozzle designs can suffer from the problems of accelerated polymer growth discussed above as well as particle growth and accumulation on the nozzle itself. U.S. Pat. Nos. 5,962,606 and 6,075,101 disclose a perpendicular catalyst spray nozzle and an effervescent catalyst spray nozzle. U.S. Pat. Nos. 6,211,310 and 6,500,905 disclose a catalyst spray nozzle having concentric tubes to flow a cleaning gas and deflecting gas along with the catalyst. Other background references include GB 618 674 A.
However, accelerated polymer growth and polymer accumulation on the nozzle remain a problem. Such particle growth and accumulation can plug the nozzle which decreases the rate of catalyst injection if not block the injection all together. As a result, production rate decreases as does the heat of reaction. Moreover, simply increasing the rate of catalyst injection or modifying monomer concentration does nothing to alleviate the problem. Instead, the nozzles are pulled from the reactor and cleaned which is disruptive to the polymerization process.
To prevent or anticipate reactor plugging, particle size data can be used to monitor catalyst dispersion. However, particle size data requires a sample from the reactor and time to analyze. This procedure is labor intensive and time consuming resulting in long lapses between sampling and the characterization of the sample. As a result of such delay between sampling and generation of data, the sample characterization provides an evaluation that lags behind the actual status of the system by several hours. In the meantime, serious fouling and agglomerates may have been generated.
There is a need, therefore, for a method for monitoring catalyst dispersion, and a need for a method for polymerization that controls catalyst dispersion based on production rate. Further, there is a need for a method for polymerization that uses a liquid catalyst and is capable of controlling polymer growth and particle size.